Add option for mapping between int64 and JS number

It would be nice if there were a compiler flag for automatically downcasting / upcasting between a JS `number` and a wasm int64. JS numbers have a precision between 32 and 64 bits (54 bits I believe). If I know my application needs more than 32 bits but less than 54 bits, it would be nice to just work with numbers rather than having to have bigints all over the place. This would have to be an opt-in option since it would cause overflow in cases where numbers exceed JS's max size.